Oilfield tubular threads typically are tapered threads on a pin member and a box member. The threads themselves may have various configurations, including a generally V-shaped or truncated V-shaped thread profile. Threads are typically tapered with respect to a central axis of the respective pin member or box member, such that the diameter of a pin thread slowly increases from a starting thread to an end thread, and similarly the diameter of the box thread slowly increases from a starting thread to an end thread. When oilfield tubular threads become worn, the threads are periodically recut or reformed to substantially conform to the original thread profile, thereby shortening the joint.
Ring and plug gauges have long been used to verify the accuracy of the threads machined on the pin member and box member. The ring gauge may thus be threaded onto a pin member, and when the gauge is hand-tight, an axial stand-off between a shoulder on the pin member and the end of the ring gauge is measured to verify that it conforms to allowable tolerances. Similarly, a plug gauge is conventionally threaded on a box member, and when the plug gauge is hand tight a stand-in is measured between an end of the box member and an end surface of the plug gauge. If measurements are within selected limits, the thread is passed for use.
In recent years, increased emphasis has been placed upon oilfield tubular threads which are multi-start threads. A dual start thread thus has two threads which typically start at circumferentially opposing locations on the pin member and the box member, with one thread being axially spaced between the other thread. In cross-section, the double-start thread on a pin member thus closely resembles a conventional single-start thread on a pin member, although in practice the double-start thread would typically make up with half the number of revolutions, thereby saving considerable rig time. Threads with three or more starts on the pin member and the box member have also been proposed. Threads with two or more starting points are considered multi-start threads.
While multi-start threads on oilfield tubular goods have been proposed for years, conventional gauging techniques have not been able to reliably test such threads. Double-start threads have previously been tested with a double-start ring gauge and a double-start plug gauge, and these gauges will result in a stand-off or stand-in as discussed above which can be measured to determine if this dimension falls within accepted limits. A significant problem, however, is that a double-start gauge may result in an acceptable stand-in or stand-off using a double start thread, but one or both of the double-start threads may still be out of tolerance even though the double-start gauge results in an acceptable stand-off and stand-in.
U.S. Pat. No. 4,356,636 discloses a gauge for a tapered threaded box of an oilfield tubular connection. U.S. Pat. No. 5,134,783 discloses thread timing for a ring gauge and a plug gauge.
The disadvantages of the prior art are overcome by the present invention, and an improved method and system are hereinafter disclosed for gauging two or more external starting threads on a pin member and two or more internal starting threads on a box member.